The present invention relates to a method and apparatus for reducing vortex-induced-vibrations ("VIV") and, more particularly, reducing VIV in marine environments by the use of fairings.
Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on fixed cylindrical marine id elements. Such marine elements are employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; cables and other mooring elements for deepwater platforms; and, although not conventionally thought of as such, the hull structure for spar type structures. These currents cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.
For short cylindrical elements that are adjacent convenient means for secure mounting, the marine elements and their supports can be made strong enough to resist significant movement by the forces created by vortex shedding. Alternatively, the marine element could be braced to change the frequency at which the element would be excited by vortex shedding.
However, strengthening or bracing becomes impractical when the application requires that the unsupported segments of the marine element extend for long runs. Deepwater production risers, drilling risers, platform export risers, import risers bringing in production from satellite wells, tendons for tension leg platforms, and other conduits for produced fluids and deepwater mooring elements formed from tubular goods are typical of such applications. These pipes and tubular goods serve as marine elements in applications that are difficult or impossible to brace sufficiently to satisfactorily control vibration induced by vortex shedding. Subsea pipelines which traverse valleys on the ocean floor can also be unsupported for extended lengths and provide another example.
Shrouds, strakes and fairings have been suggested for such applications to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the orientation of the current to the marine element. But shrouds and strakes are generally less effective than fairings. Fairings are generally very effective in reducing vibrations due to vortex shedding, and also reduce drag forces on the marine element. Until recently, however, fairings have been thought to be effective only if the relative direction of the current is parallel to the axis of the fairing.
U.S. Pat. Nos. 4,398,487 and 4,474,129 disclose conventional, long fairings for use with subsea pipes and risers which are provided with means to permit the fairing to rotate around the pipe or riser as would a weathervane in order to maintain an orientation presenting the fairing parallel to the current. Accommodating this constraint results in an expensive apparatus.
Further, the subsea environment in which the fairings must operate renders likely the rapid failure of the rotational elements. Traditional fairings present a chord to thickness ratio greater than two and present a very serious problem should corrosion or marine growth cause the rotational elements to seize up. Such a failure of the fairing to rotate would cause excessive drag forces on the marine element should the current shift and no longer align with the "frozen" fairing. As a result, rotatable fairings have, in actual practice, been limited to drilling riser applications in which the risers (together with fairing mounted thereon) are frequently and routinely retrieved and not left in service for extended periods.
U.S. Pat. No. 5,410,979 represents a significant breakthrough in providing for fixed, short fairings. Even so, short fairings having a maximum ratio of length to width (cylinder diameter) of from 1.5 to as low as about 1.25 still drive cost and otherwise limit application. This length to width ratio, as a practical matter, translates to about a 1.20 minimum chord to thickness ratio for these short fairings.
These limitations are particularly constraining in the case of large diameter marine elements, such as spar hulls, which have conventionally been considered unsuitable for fairings, yet are subject to VIV problems as moored at the distal end of long tethers and/or mooring lines which provide no suitable means for bracing support.
Thus there remains a need for improved fairings that are effective to reduce vibration of a substantially cylindrical marine element caused by vortex shedding. An advantage of the ultrashort fairings of the present invention is less material is used and that the material requirements are relaxed with the shortening of the fairing and the reduced drag effects on fixed fairings. Another advantage of ultrashort fairings is that they facilitate pipe handling in substantially conventional manners not permitted with longer fairings. Further, in non-conventional fairing applications, e.g., spar hulls, ultrashort fairings enable enhanced practical application. Other embodiments may also benefit from new practicality in forming the walls of marine elements in a shape including an ultrashort fairing profile. Further, ultrashort fairings can increase effective VIV suppression across a wider range of angles of attack. Other deployment of embodiments of the present invention provide fixed fairings of sufficiently limited drag to enable an array of fixed fairing orientations to be simultaneously deployed on a single marine element to broaden the environmental conditions that will meet adequate VIV suppression.